1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser array applicable to light sources of optical interconnections, optical memories, optical switches, optical information processing, laser beam printing machines and copying machines, and an optical transmission system using the array.
2. Description of the Related Art
A vertical cavity surface emitting laser diode (hereinafter referred to as VCSEL) is an optical device that has a resonator formed in the direction vertical to the semiconductor substrate, and emits light in this vertical direction. Multiple VCSELs are attractive as parallel-arranged optical sources that can be arranged in a one-dimensional or two-dimensional array in highly integrated fashion.
In VCSEL, the carrier and light are confined in the vertical cavity in order to efficiently emit laser light. There are several means for realizing a confinement structure in the horizontal direction of the substrate, and examples of these means are typically of air post type, selective oxidization type, proton projection type and proton reflection type. The air post type has a slender post on the substrate and the post itself functions as a current path. In the selective oxidization type, a post structure is formed on the substrate, and a part of an AlAs layer called a control layer is oxidized so that a limited current path can be formed. In the proton type, a limited current path is formed by defining an insulation region by proton implantation. The selectively oxidization type of VCSEL among the various types of VCSELs has a low threshold current and excellent light-current characteristics. Thus, there is considerable activity in the reduction to practice.
Japanese Patent Application Publication 2004-23087 discloses a VCSEL in which the side surfaces of a contact layer, an upper multi-layered film reflection mirror and a current confinement portion are aligned with a metal contact layer provided on the tope of the mesa. This arrangement improves the precision in alignment of the metal contact layer having a light-emitting aperture with the current confinement portion, and realizes improved optical output although laser oscillation is in the fundamental lateral mode.
The light source of special transmission needs comparatively large power. Thus, a VCSEL of multi-spot type is used in which multiple light-emitting units (multiple spots) serving as parallel-arranged light sources are formed on the substrate. The multiple spots are driven by identical drive signals from a drive circuit, and laser lights emitted from the multiple spots are combined.
Japanese Patent Application Publication No. 10-65266 discloses a VCSEL in which at least one of upper and lower semiconductor multi-layered reflection mirrors is made longer than the other in a direction on the plane of the semiconductor substrate so that the emitted beam has a directional intensity pattern. This application describes a laser device, as exemplarily shown in FIG. 20, in which the VCSELs are one-dimensionally or two-dimensionally arrayed so that the short axes of light emission patterns having a rectangular or oval shape are aligned in a direction of n. This makes it possible to realize uniform light emission.
Japanese Patent Application No. 10-52941 discloses a light source, as exemplarily shown in FIG. 21, in which light sources are arrayed so that the light emission apertures (near field pattern sizes) become smaller from the center of the main scan direction Y towards opposing ends of the array. The light beams emitted from the light sources in the center form an image at a focusing point on the plane of a photosensitive member. In contrast, the light beams emitted from the light sources in the ends form an image at a focusing point short of the photosensitive member, and gradually become thick. Then, these beams form a spot size as large as the spot size available in the center on the plane of the photosensitive member.
However, the above-mentioned multi-spot type VCSELs have the following drawbacks. FIG. 22 schematically shows two adjacent light-emitting portions of the conventional multi-spot type VCSEL. Referring to this figure, on an n-type semiconductor substrate 910, provided are an n-type lower reflection mirror 911, an active layer 912, a p-type current funneling portion 913, and a p-type upper reflection mirror laminated in this order. A p-side electrode layer 915 is formed on the laminate. Cylindrical posts (mesas) 916 are formed on the substrate, and include the upper reflection mirror 914, the current funneling portion 913, the active layer 912 and the lower reflection mirror 911. A selectively oxidized region 913a is formed in an outer peripheral portion of the current funneling portion 913 in each post 916. The remaining non-oxidized portion is an aperture 913b that has a circular shape and a conductive region. The p-side electrode layer 915 has circular light emission apertures 917, which are respectively aligned with the centers of C1 and C2 of the apertures 913b in the axial directions. An n-side electrode 918, which is provided common to the posts (light-emitting portions) 916, is provided on the backside of the semiconductor substrate 910.
When the VICSEL is driven, laser lights having a divergence angle θ with respect to the center axes C1 and C2 are emitted from the substrate 910 in the vertical direction in accordance with the diameters of the apertures 913b and those of the light-emission apertures 917 in the electrode layer 915. At that time, near field patterns (NFP) 920 in the light-emission apertures 917 of the posts 916 have symmetrical single peaks, and a near field profile 921 of the combined laser beam has a portion 922 in which power is reduced. This is because there is a difficulty in close arrangement of the posts 916 over a given interval due to fabrication limitations. The power-reduced portion 922 is not good in optical systems over which light is transmitted.
When laser light is incident to a light slit, a lens and a light-receiving element, the divergence angle of the laser light is preferably stable and narrow. The single-spot VCSEL may easily meet the above requirement. In contrast, the multi-spot type laser array has difficulty in reduction of the spot intervals over the given distance due to the fabrication limitations as described above. The light-emitting points of the laser lights are spatially spread, and the divergence angle of the combined laser beam has difficulty in narrowing as much as that of the single-spot. Especially, the laser beams from the posts (light-emitting portions) located in the array ends are made dark through an optical member such as a lens. This makes it difficult to reduce the divergence angles of the laser beams from the array ends.
The emission near field patterns described in Japanese Patent Application Publication No. 10-65266 have drawbacks similar to those mentioned above. The near field patterns described in Japanese Patent Application No. 10-52941 are symmetrical although the light-emission apertures have different sizes. The combined laser field has a near field pattern or beam profile that has an output reduced portion in the center and cannot eliminate the above-mentioned drawbacks.